Communication system determining time of arrival using matching pursuit

ABSTRACT

A wireless receiver receives location pilots embedded in received symbols and uses the location pilots to detect the first path for every base station the network has designated for the receiver to use in time of arrival estimation. The receiver preferably applies matching pursuit strategies to offer a robust and reliable identification of a channel impulse response&#39;s first path. The receiver may also receive and use estimation pilots as a supplement to the location pilot information in determining time of arrival. The receiver can use metrics characteristic of the channel to improve the robustness and reliability of the identification of a CIR&#39;s first path. With the first path identified, the receiver measures the time of arrival for signals from that path and the receiver determines the observed time difference of arrival (OTDOA) to respond to network requests for OTDOA and position determination measurements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/930,323, filed May 12, 2020, entitled COMMUNICATION SYSTEMDETERMINING TIME OF ARRIVAL USING MATCHING PURSUIT, which is acontinuation of U.S. application Ser. No. 14/811,507, filed Jul. 28,2015, entitled COMMUNICATION SYSTEM DETERMINING TIME OF ARRIVAL USINGMATCHING PURSUIT, Issued U.S. Pat. No. 10,749,778, the disclosures ofwhich are incorporated herein by reference in their entirety for allpurposes.

BACKGROUND 1. Field of the Invention

The present invention relates to a system and method for determiningposition of a user terminal or other communication equipment based ontime of arrival measurements in a wireless environment.

2. Description of the Related Art

Measurements of times of arrival (TOA) for signals from a set ofwireless base stations can aid in determining a user's position orlocation. For example, an LTE (long term evolution) standard receivercan determine its location (or positioning) based on LTE signaling in away that can replace or supplement GPS or WiFi-assisted positioningstrategies.

The LTE positioning protocol, described for example in ETSI TS 136 355version 10.0.0, which is referenced in release 10 of the LTEspecification, embeds positioning reference signal (PRS) subcarriersinto designated orthogonal frequency division multiplexing (OFDM)symbols over specified time intervals, sometimes called positioningoccasions. The user equipment (UE) may measure the time of arrival (TOA)of PRS subcarriers from each accessible base station (which the LTEspecification calls the eNodeB). The user equipment preferably measuresat least one reference signal time difference (RSTD) between twodifferent eNodeBs (one called the reference and the other called theneighbor). The reference signal time difference is related to theestablished measure for observed time difference of arrival (OTDOA)described in the LTE positioning protocol.

The reference signal time difference measurement is simple in concept.In practice, the measurement of any TOA from an eNodeB can be unreliablebecause of errors due to the severe multipath environment in wirelessnetworks and the typically low signal-to-noise ratio (SNR). Theincreased density of base stations and users in an LTE network alsoincreases the potential for measurement errors. Determining the positionof user equipment proceeds by measuring the TOA of the first path fromeach eNodeB of interest followed by determining the reference signaltime difference (RSTD) between pairs of designated eNodeB base stationsusing the respectively measured times of arrival at the user equipment.Depending on the particular configurations specified in the standard,the TOA and RSTD measurements may be made over a specified number ofbase stations and different corresponding combinations of RSTDmeasurements between ones of the set of base stations.

The difficulties that arise in measuring TOAs relate to identifying thefirst path arriving at a user equipment terminal (UE) from anydesignated eNodeB. It is common for a wireless channel's impulseresponse to consist of a small plurality of paths at varying amplitudesand delays relative to a first path. The strongest path in a wirelesschannel's impulse response may not be indicative of the true delay,since the first path may be of lower amplitude than the strongest path.In addition to the fact that the first path might not have the greatestamplitude, identification of the first path can be complicated by thelikelihood of falsely identifying paths due to correlation withinterference, high levels of noise, or both.

LTE's fundamental modulation scheme to transmit bits over the air usesOFDM. That is, bits are generated by applying quadrature amplitudemodulation (QAM) to each active subcarrier that makes up an OFDM symbol.In practice, an LTE OFDM symbol may have 1024 time samples representing600 active subcarriers out of a maximum of 1024 subcarriers. Eachsubcarrier may be assigned a function at the receiver, such astransmitting bits known a priori to the receiver and thus enablingdifferent calculations. These calculations may include channel impulseresponse (CIR) estimation and positioning-related measurements.

FIG. 1 provides a functional block diagram of apparatus for determiningposition using observed time difference of arrival (OTDOA) based on thereference signal time difference (RSTD) measurement specified in LTE.The illustrated user equipment receiver 110 receives a plurality of OFDMsymbols from two base stations 101, 103. Receiver 110 may use one ormore antennas to receive the symbols. FIG. 1 illustrates the positiondetermining functionality using as an example signals received from twobase stations 101, 103 with the receiver 110 using a single antenna,which is the minimum configuration for an RSTD measurement. Thisconfiguration can be extended to a greater number of base stations and agreater number of user equipment antennas.

Because user equipment receiver 110 is compliant with the LTE standard,the receiver can process received OFDM symbols to provide best estimatesof the transmitted bits. Such a receiver 110 can identify the first pathusing one or more first path identification (FP-ID) modules 130, 140,which are responsive to subcarriers assigned to calculate positioninginformation. Each first path identification module 130, 140 isresponsive to information 132, 142 provided by the user equipmentreceiver 110 about the subcarriers to be used for positioningmeasurements. For example, the information may be stored within tablesin non-volatile memory.

The first path identification units 130, 140 identify the respectivefirst path for the received OFDM symbols from a known eNodeB. Thereference signal time difference (RSTD) measurement typically is basedon a predetermined duration of OFDM symbols to achieve the desiredaccuracy. In LTE, this may be over at least one sub-frame of OFDMsymbols, which is specified to be fourteen OFDM symbols.

The output from each first path identification module 130, 140 is thetime of arrival (TOA) at the user equipment of a signal from thecorresponding base station. Generally, in LTE, the RSTD_(k,j) betweenbase stations indexed as k and j is determined as

RSTD_(k,j)=TOA_(k)−TOA_(j).  EQ. 1

FIG. 1 shows that module 150 of the receiver 110 provides as its output152 the equation 1 reference signal time difference computation. Thisoutput RSTD_(0,1) 152 is the output 134 of first path identificationmodule 130 minus the output 144 of first path identification module 140.

The calculation of RSTD_(k,j) is simple given a reliable estimation ofthe TOA_(k) and TOA_(j), knowing the structure of the signal received atthe first path identification FP-ID module 130, 140. The standard, suchas the LTE standard, specifies the structure of the symbol, which can begeneralized as shown in FIG. 2. Modern wireless systems that transmitfrom one source, such as a base station, to multiple users in thecoverage area, require the transmission to be subdivided into“subchannels.” This is not much different in concept than FM or AM radiotransmission; however, wireless transmission has a goal of very highbits/Hertz for a given spectrum. In the case of modern wirelesstechnology, specifying channels can be achieved using orthogonalschemes, which include OFDM and code division multiple access (CDMA). Inthe near future the wireless standards may increase capacity by usingquasi-orthogonal channels achieved in myriad spatial and temporalstrategies.

FIG. 2 simplifies the explanation of the signals involved in an observedtime difference of arrival (OTDOA) measurement by showing a method thatassumes orthogonal channelization. That is, while orthogonality isretained, the crosstalk between channels is kept to insignificantlevels.

The FIG. 2 horizontal axis 201 represents time, qualitativelyrepresenting the time occupied by received symbols. FIG. 2's verticalaxis shows a second channel dimension such that FIG. 2 qualitativelyshows channels as having no overlap. The vertical axis channelseparation can represent segments of frequency, as in the case of OFDM,or the indexing of different codes in CDMA. For example, in the presentLTE standard, the segmentation in the frequency axis can represent 15kHz of bandwidth for a subcarrier, with an OFDM symbol possiblyconsisting of up to 600 active subcarriers out of 1024 total subcarriersin one symbol. This is only an example and other allocations are known.Thus, for example, the extent of each square in FIG. 2 can represent 15kHz (y-axis) by 71.4 μs (x-axis) in the frequency-by-time grid. Thevalue 71.4 μs is determined by the 1000 μs duration of an LTE sub-framedivided by the number 14 of OFDM symbols specified as making up an LTEsub-frame. In LTE terminology, each 15 kHz (y-axis) by 71.4 μs (x-axis)block in the grid is called a resource block (RB).

The following discussion of FIG. 2 focuses on OFDM transmission, but itshould be appreciated that FIG. 2 can equally illustrate othertransmission systems. For example, FIG. 2 can illustrate otherorthogonal schemes such as CDMA transmission as well as quasi-orthogonaltransmission strategies such as those proposed for next generationwireless (5G). The orthogonal or quasi-orthogonal transmissionstrategies might be used for subchannels or for signaling related toobserved time difference of arrival (OTDOA) measurements, among othertransmission strategies.

To allow for user equipment terminals to determine position by computingOTDOA, certain wireless standards assign subcarriers in the grid to beused for determining position or accomplishing OTDOA functionality. Tosimplify this discussion, exemplary OTDOA subcarriers are designated as“location pilots” (LP) 212, 214, 216 in FIG. 2. The term pilot is usedto denote a subcarrier with a known transmit modulation at the receiver.These pilot subcarriers are in contrast to data subcarriers, which haveunknown modulation characteristics because they are encoded with unknowninformation bits. This pilot scheme allows a compatible terminal toaccomplish various measurements. User equipment terminals generally needto estimate the channel impulse response (CIR) and other parameters forsuccessful reception and demodulation of OFDM symbols. Consequently, thegrid shown in FIG. 2 likely contains other subcarriers designated aspilots. These persistent pilots are denoted as estimation pilots (EP)and are indicated as 221, 223, 225 in FIG. 2. In LTE, these locations,and EP modulating bits, are known at the receiver because they aredictated by the LTE specification.

While the user equipment is guaranteed to receive the location pilots atthe time of a request for OTDOA measurement, the number of estimationpilots present will vary by configuration of the user equipment in thenetwork. In the context of LTE, the location pilots are designated aspositioning reference signals (PRS), and the estimation pilots aredesignated as common reference signals (CRS). In the LTE scheme, theuser equipment receiver will always receive at least one set of CRSsubcarriers in a sub-frame, and possibly may receive additional sets ofsubcarriers. It is reasonable to consider that a user equipment receivercan receive up to four CRS sets in a sub-frame. In the nomenclature ofLTE, these sets are called “antenna ports.” Normally, CRS subcarriersare used for channel impulse response (CIR) estimation, among otherparameter estimations. Another property of the grid shown in FIG. 2, asit applies to LTE, is that the CRS and PRS are distributed on the600-by-14 grid, and need not be contiguous in time or frequency. FIG. 2shows arbitrarily positioned location pilots and estimation pilotsconsistent with this observation.

SUMMARY OF THE PREFERRED EMBODIMENTS

An aspect of the preferred embodiments provides a method of determiningtime of arrival in a wireless network. The method includes receiving ata receiver from a wireless network a request for time of arrivalinformation, extracting location pilot symbols from a received signal,identifying a first path associated with the location pilot symbolsusing matching pursuit and determining a time of arrival based on thefirst path identified using matching pursuit. The receiver thencommunicates information responsive to the time of arrival based on thefirst path from the receiver to the wireless network.

Another aspect of the present invention provides a receiver with a timeof arrival function. The receiver including a memory coupled to storeextracted location pilot symbols and an interpolator coupled to receivethe extracted location pilot symbols and to generate virtual locationpilot symbols. A multi-symbol correlator correlates the extractedlocation pilot symbols and the virtual location pilot symbols with areference signal and outputs an initial location pilot channel impulseresponse. A matching pursuit estimator providing a channel impulseresponse estimate responsive to the initial location pilot channelimpulse response. A time of arrival estimator responsive to the matchingpursuit estimator to identify a first path and to determine a time ofarrival for the first path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of apparatus to perform an observed timedifference of arrival (OTDOA) measurement between two base stationsusing a single antenna receiver.

FIG. 2 shows a nominal grid of subchannels used in a method for wirelesstransmission of data and calibration information.

FIG. 3 shows a functional block diagram to extract the location andestimation pilots from a received sequence of symbols

FIG. 4 shows the matching pursuit channel impulse response (MP-CIR) timeof arrival estimator used to estimate the time of arrival for eachantenna and each LP (EP) symbol individually.

FIG. 5 shows the matching pursuit channel impulse response (MP-CIR) timeof arrival estimator used to estimate the time of arrival for eachantenna and for each location or estimation pilot symbol individuallywith further aid from a time domain channel estimator.

FIG. 6 shows the matching pursuit power delay profile (MP-PDP) time ofarrival estimator used to estimate the time of arrival for each antennaand each location or estimation pilot symbol individually.

FIG. 7 shows the matching pursuit power delay profile (MP-PDP) time ofarrival estimator used to estimate the time of arrival for each antennaand each location or estimation pilot symbol individually with furtheraid from a time domain channel estimator.

FIG. 8 shows another aspect of a receiver and, more particularly, aninterpolation module that can be used when desired to provide greatergranularity to the matching pursuit estimate, for example, to providefractional estimation of the time of arrival estimate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspects of the present invention provide communication apparatus andcommunication methods for measuring time of arrival (TOA) at a wirelessreceiver. A wireless network operating with a preferred wirelessreceiver according to a given standard may use this time of arrivalmeasuring system to determine the receiver's position. The preferredwireless receiver preferably reacts to symbols designated in accordancewith the standard to perform time of arrival measurements. A preferredreceiver may, for example, measure time of arrival values and use thosevalues to determine an observed time difference of arrival (OTDOA)between transmitting base stations, which can be transmitted to thenetwork to provide position information relative to the base stations.

In accordance with other aspects, a system or method preferably maymeasure time of arrival from a plurality of transmitted pilots, definedas subchannels and having modulation a priori known to the receiver.These transmitted pilots may preferably include pilots or signals otherthan the location pilot signals specified by the standard for use inmeasuring time of arrival.

A preferred implementation of a time of arrival measuring system ormethod uses one or more strategies for estimating the first path fromeach designated base station. These strategies may, for example, includeapplications of iterative matching pursuit (MP) to identify the firstpath in a wireless channel's impulse response for each transmitting basestation assigned to a receiver.

In accordance with other aspects, a preferred system or method maycontrol matching pursuit iterations to improve the robustness andreliability of the first path identification. This system or method mayevaluate metrics to determine when interference or noise may bedetrimental to the identification of first paths and may take responsiveaction to improve the quality of the first path identification andinformation. This system or method may also use information from a timedomain channel estimator to identify or characterize the interference ornoise.

A system or method for determining position based on measuring time ofarrival uses a signaling method embedded into a multiple access schemeand a receiver that can measure the first path in the channel impulseresponse (CIR) to compute the reference signal time difference (RSTD)between any given pair of base stations.

The time of arrival is preferably determined from the first pathmeasurement using the knowledge that the wireless standards define andtransmit symbols with a rigid timing structure. For example, in longterm evolution or LTE systems using orthogonal frequency divisionmultiplexing (OFDM) symbols, the OFDM symbols are grouped into subframesand each subframe can last 1000 μs in one common configuration. Eachsubframe is divided into two slots, each lasting 500 μs, and each slotcontains seven OFDM symbols. Each slot has a defined structure ofsamples that make up the OFDM symbols and additional overhead samples.The receiver knows the time at the beginning of each slot throughsynchronization schemes detailed in the LTE standard. Each sample isknown to be of six microseconds duration. Thus, with the time known forthe beginning of a slot, the first path determines the offset durationin terms of samples, or a scale thereof, from the beginning of theestablished time for the associated processed slot or slots. In turn,this determines the time of arrival at the receiver. That is, the timeof arrival can be determined from the first path identification byreferring to the time scale set by the subframe start, counting thenumber of samples from the time reference until the first path andconverting that to a time using the sampling rate. As discussed below,it is also possible to make a fractional determination between sampletimes when greater accuracy for time of arrival or receiver position isdesired.

A system or method for determining times of arrival preferably reacts tothe presence of pilots embedded in the received symbol to detect thefirst path from each base station assigned to a receiver (userequipment) for time of arrival estimation. An exemplary system or methodpreferably applies matching pursuit (MP) strategies to offer a robustand reliable identification of a channel impulse response's first path.Further aspects of these matching pursuit strategies include controllingthe matching pursuit iterations to mitigate degradations due tointerference or noise. Other aspects may advantageously react toinformation from a time domain channel estimator to improve upon therobustness and reliability of the identification of a CIR's first path.With the first path identified, the receiver measures the time ofarrival for signals from that path and the receiver's observed timedifference of arrival (OTDOA) estimator responds to network requests forOTDOA measurements and position determinations.

Any wireless receiver must extract specified subchannels from a receivedsymbol according to an established standard as part of recoveringtransmitted information bits. Consequently, the typical wirelessreceiver naturally performs an estimation pilot extraction processsimilar to that used for extracting location pilots (LP) in the processof measuring time of arrival. Preferred embodiments of a wirelessreceiver preferably extract location pilots and particularly preferredreceivers further also extract estimation pilots (EP) so that both LPand EP information can be used to improve estimation accuracy. FIG. 3shows how the receiver extracts location and estimation pilotinformation from received OFDM or other symbols. A receiver as shown inFIG. 3 preferably extracts and provides time domain symbols 372consisting only of LP subchannels, with other subchannels removed.Further preferred embodiments can provide time domain symbols consistingonly of EP subchannels 362, with other subchannels removed. Thesefiltered symbols simplify and so facilitate the time of arrivalmeasurements.

A wireless receiver incorporates circuitry to receive transmissions froma base station with at least one antenna 300. Analog circuitry 310conditions the received signal and provides a digital representation,via quantization, of the received analog waveform. While FIG. 3 shows asingle antenna 300 and corresponding analog circuitry 310, otherimplementations provide replicas of antenna 300 and analog circuitry 310to provide the receiver with a plurality of antennas and correspondingfront end circuitry to advantageously increase the receiver performanceusing the resulting plurality of antennas. Whether in a single antennaor multiple antenna configuration, the receiver's front end circuitrysuch as 310 outputs a digitized signal that is then processed throughcorresponding receiver processing circuitry 320 to acquire thetransmitted symbol with the highest signal-to-noise ratio (SNR), forexample, by using well-known processing.

Estimating the time of arrival using the FIG. 3 receiver does notrequire demodulation of the location pilot (LP) or, if present,estimation pilot (EP), as must be done for information-bearingsubcarriers. Regardless, the receiver preferably extracts the LPs and,if present and they are to be used, the EPs from the incoming symbol by“zeroing” all other subchannels. The utility of “zeroing” all othersubchannels is illustrated by an example of the OFDM modulation used inthe LTE standard. An OFDM receiver has knowledge of the LP and EPsubcarriers present in any given symbol, for example, among a possible600 candidate subcarriers within a 1024 sample symbol. The OFDM receiverdoes not have knowledge of the other (non-pilot) symbols and so cannotdetermine how those other symbols were affected by the channel. Combfiltering 340, 350 can be applied to frequency domain representations ofthe respective symbols, whereby all frequency subcarriers not identifiedas LP subcarriers are given a value of zero, and the 1024 sample symbolis created with only LP subcarriers active as having their receivedvalues. This procedure is repeated for the subcarriers when EPsubcarriers are present and are desired to be used. This allows thereceiver to process all of the pilot symbols about which it hasinformation, without interference from the unknown informationassociated with the other (non-pilot) symbols.

Referring to FIG. 3, the receiver processing circuitry provides thedigitized and acquired symbols to dechannelizer circuitry 330, whichprocesses the digitized and acquired symbols so that they can berecognized and processed. The dechannelizer circuitry 330 presents allactive subchannels in parallel to the location pilot (LP) comb filter350, which zeroes all non-LP subcarriers. The LP comb filter 350provides the processed output including the extracted LP subcarriers totime converter 370 that converts the processed output to a time domainsymbol, LP symbol 372. For OFDM symbols, the dechannelizer circuitry 330performs a fast Fourier transform (FFT) on the input OFDM time samples,and the time converter 370 is an inverse FFT. Comb filtering is mostefficiently performed in the frequency domain for the illustrative OFDMwaveform. Corresponding strategies to extract the LP and estimationpilot (EP) subcarriers from signals for other transmission schemes, suchas CDMA and quasi-orthogonal channeling, are alternately implemented incorresponding receivers for those technologies. The output from the timeconverter 370 is an LP symbol 372, which has the structure of a timedomain OFDM symbol having a number of samples to make it equal in lengthto the symbol input to the dechannelizer circuitry 330. The LP symbolonly has information about the location pilots. Likewise, the outputfrom the time converter 360 is an EP symbol 372, which has the structureof a time domain OFDM symbol having a number of samples equal in number(length) to the symbol input to the dechannelizer circuitry 330.

In the specific case of LTE transmission, the receiver front endprocessor 320 performs the acquisition of timing and strips the cycleprefix required for OFDM modulation from the received samples. Thedechannelizer circuitry 330 is an FFT, and the LP comb filter 350 zeroesall non-PRS (positioning reference signal) subcarriers. Time converter370 takes an inverse FFT of the samples provided to it. Similarfunctions are performed by the EP comb filter 340, which preserves thecommon reference signal (CRS) subcarriers and zeroes other subcarriers,and the time converter 360, which performs an inverse FFT on the outputof the comb filter 340 so that EP symbol 362 is output in the timedomain.

In some implementations, the FIG. 3 and other preferred receivers reactat least to the LP symbol 372, and preferably also simultaneously reactto the EP symbol 362. For an LTE implementation of the FIG. 3 receiver,the receiver preferably reacts to at least the PRS symbol represented byLP symbol 372, and preferably simultaneously reacts to the CRS symbolrepresented by EP symbol 362. For an LTE implementation of the FIG. 3receiver, the receiver may also react to at least the CRS symbolrepresented by EP symbol 362, particularly when the PRS symbolrepresented by LP symbol 372 is absent. For receivers that have multipleantennas, the receivers preferably perform the procedures illustrated inFIG. 3 for each of the receivers' physical antennas. If two or moreantennas are present, then there will be an LP symbol 372 for eachantenna. The same will apply to the EP symbol 362, so that there will beone such signal for each antenna.

The above and following descriptions focus on determining time ofarrival and determining position using location pilots as a primaryresource while also using estimation pilots and other information assupplements to the location pilot symbols. There are situations where itis preferred to use only estimation pilots or, at least, to useestimation pilot information as a primary resource for time of arrivalmeasurements. For example, there are important situations in LTE systemswhere the network or the receiver preferably determines time of arrivalestimates only from CRS (estimation) pilot symbols. This is particularlytrue, at least for symbols or situations where location pilotinformation is not available, but is also more generally true. Onespecific application that benefits from using CRS pilot symbols for timeof arrival measurements is in the receiver (user equipment) assistedpositioning technique where the receiver reports a receiver-determinedtime of arrival to the network and then the network determines thereceiver location. Another application is when the receiver determinesits location without network assistance with the receiver estimating thetime of arrival (distance) of surrounding eNodeB s (base stations) andthen solving the navigation equations locally. For this application, thereceiver needs to know the coordinates of the eNodeBs. It can also bedesirable for a receiver (user equipment) to determine a CRS-based timeof arrival to provide fine timing for standard baseband receiverprocessing (to recover user bits, decoder control messages, etc.).

FIG. 4 shows an implementation of a receiver that estimates time ofarrival using a matching pursuit strategy. The FIG. 4 implementation maybe considered as including a “stream” time of arrival estimator, wherestream signifies that estimation is performed using location pilot (LP)or estimation pilot (EP) subchannels received at one antenna in thereceiver from a single base station. In the FIG. 4 implementation, eachof the available streams of LP symbols 372 and EP symbols 362 isprocessed individually with a matching pursuit (MP) iterative strategyto identify the first path in the channel impulse response (CIR) and toestimate the time of arrival (TOA).

The FIG. 4 receiver includes a queue 400 that stores the LP 372 symbols,EP symbols 362 or both for the selected antenna(s) and pilot type(s) thereceiver uses for the TOA estimation. The LP 372 and EP 362 symbols areextracted as discussed above and as illustrated in FIG. 3. Theparticular symbol for TOA measurement is denoted in FIG. 4 genericallyas the received pilot symbol 402. Preferably the receiver is configuredto provide one or more sets of the circuitry shown in FIG. 4 asappropriate to the antennas and desired pilots, for example, providingone replica of the FIG. 4 circuitry per stream of pilot symbol andantenna. Alternatively, the receiver illustrated in FIG. 4 couldimplement a time-sharing method for processing multiple streams of pilotsymbols using partially or completely common circuitry, depending on therequirements and computational delays in estimating all times of arrivalfor signals from all of the base stations.

In a preferred implementation, the FIG. 4 receiver includes a pilotinterpolator 410 that can provide pilot information for instances whenthe received symbol does not contain a pilot. For example, in the caseof LTE, not all 14 symbols in a subframe have LPs or EPs present. Thus,the pilot interpolator 410 preferably determines virtual pilots forsymbols where an LP or EP is not present. The process of interpolatinguses the values of pilots from other symbols to estimate the value of a“virtual” pilot positioned within the desired symbol. Further discussionof interpolation and appropriate interpolation circuitry is discussed inU.S. Pat. No. 8,897,353, which is incorporated here by reference in itsentirety and for all purposes. Preferably the interpolation strategy isselected for the given performance and pilot density to achieve thetarget first path identification (FP-ID) performance. The receiverpreferably performs pilot interpolation calculations to generate virtualpilots for each of the received symbols so that the receiver provides alocation or estimation pilot value (actual or virtual) in each segmentfor determining a time of arrival. Preferably the pilot density isselected to provide sufficient signal quality to allow for time ofarrival measurements to be made in the expected signal to noiseenvironment for receiver operation.

The multi-symbol correlator 420 performs time domain correlation betweenthe concatenated and interpolated time domain signal p[n] 412 and alocally provided reference signal r[n] 414. p[n] 412 includes the combfiltered time domain LP and, when used, EP signals concatenated into astream and preferably including added virtual pilot signals. Thereceiver generates or stores r[n], which is generated based on knowledgeof the (standard-dictated and interpolated) LP and EP pilot positionsand their intended modulation to produce an idealized set of expectedpilot values and positions. The correlation between p[n] 412 and r[n]414 would produce a single spike at the time alignment if the receivedsignal p[n] 412 had its idealized form created by passing through anidealized channel and received by an ideal receiver. This idealizedcorrelation can be represented as:

t[n]=Σ_(l=0) ^(L) p[l]r*[l+n].  EQ. 2

The correlation of equation 2 can be determined in various ways,generally with a goal of reducing computational complexity, given thelength of the correlation t[n] and the concatenated symbol lengths L.For example, the correlation may be determined using fast Fouriertransform strategies. Preferably the multi-symbol correlator 420performs a correlation over multiple OFDM symbols containing locationpilot symbols 372, estimation pilot symbols 362 or both. The practical,real channel correlation performed in the multi-symbol correlator 420will not be ideal and will require further processing to accuratelycharacterize the pilots and the channel over which they weretransmitted. That further processing is preferably performed usingmatching pursuit strategies.

Various schemes are defined in standards that allow wireless receiversto determine the values present in the LP and EP subchannels. Thesestandard-dictated values are stored local to the receiver and, with theinterpolated pilot information, determine the corresponding referencepilot symbols 404 that are stored in the queue 405. The multi-symbolcorrelator 420 receives r[n] and p[n] and performs correlation for theduration of the symbols designated for the time of arrival measurement,which in the case of LTE may have a duration of fourteen symbols. Theoutput t[n] 422 of the correlator 420 is then provided to the matchingpursuit channel impulse response estimator 430, which evaluates thechannel impulse response using matching pursuit techniques and outputsan improved channel impulse response estimate. More specifically, thematching pursuit estimator 430 receives the correlation output bycorrelator 420 as an input to the matching pursuit process and theestimator 430 processes the correlation output t[n] 422 to extractchannel impulse response information from the correlation output t[n]422. Estimator 430 thus determines a channel impulse response andoutputs an identification of the first path T_(1stPath) 432 that allowsthe time of arrival estimator 450 to determine the relative time delayof that first path. The time of arrival estimator 450 may, for example,establish a time reference 482 at the beginning of the slot (subframe orframe) 460 relevant to the observed first path and then identify (countfrom the time reference) the sample associated with the first path andestablish the time of arrival 452 from the sample count and the samplingrate.

Matching pursuit is an effective strategy for detecting the presence ofsignals that contain properties stored in a dictionary 440. Matchingpursuit iterates through a process to reconstruct a signal with aminimum of dictionary “words.” The pseudo-code for the matching pursuitprocedure can be as follows:

Matching Pursuit Pseudocode

-   0. define a dictionary to represent the signals of interest;-   1. candidate_signal=0;-   2. observed_signal=input_signal;-   3. while stopping_criterion is not met do:    -   3.1 find best_dictionary_word for the working_signal;    -   3.2 candidate_signal=candidate_signal+best_dictionary_word;    -   3.3 observed_signal=observed_signal−scaled best_dictionary_word;    -   3.4 compute stopping_criterion;-   4. end while.

Applying this framework to the FIG. 4 receiver, the receiver identifiesthe correlator output t[n] 422 as the input_signal in step 2 and thedictionary 440 is based on the pilot reference r[n]. As evident fromstep 3.4 of the matching pursuit pseudocode, it is important to find agood stopping criterion that does not end prematurely and does notcontinue beyond the level needed to establish a desired level offidelity for the candidate_signal.

Preferably a receiver or receiving method implements matching pursuitusing a stopping criterion based on the estimated signal to noise ratio,as determined by the SNR metric 470 from either the unprocessedcorrelation t[n] output by the multi-symbol correlator 420 or theprocessed and improved multi-symbol correlation t_(M)p[m] 474 outputfrom the matching pursuit processor 430. For an initial state ofreceiver operation, the SNR metric processor 470 preferably determinesan initial signal to noise ratio from the first multi-symbolcorrelation. For example, an initial signal to noise ratio might bedetermined from:

$\begin{matrix}{{{SNR}\lbrack 0\rbrack} = {\frac{{{\max\left\{ {t\lbrack l\rbrack} \right\}}}^{2} \cdot \left( {L + 1} \right)}{\sum_{l = 0}^{L}\left( {{{t\lbrack l\rbrack}}^{2} - {{\max\left\{ {t\lbrack l\rbrack} \right\}}}^{2}} \right)}.}} & {{EQ}.\mspace{14mu} 3}\end{matrix}$

For subsequent iterations, the improved multi-symbol correlationt_(MP)[m] 474 is preferably substituted for each instance of the initialmulti-symbol correlation t[n] in the equation 3 calculation and thesignal to noise ratio is calculated from that modified equation. Thissignal to noise metric can be viewed as the ratio of power in the signalpaths to the power in the no-signal paths.

As discussed above, the MP-CIR estimator 430 preferably implementsmatching pursuit to identify the paths and delays in the receivedsignals using the output of the correlator 420 and the dictionary 440.In the FIG. 4 receiver, the matching pursuit process preferably followsthe pseudocode as stated above, with the dictionary 440 preferablyconstructed based on the multiple-access scheme for each subchannel. Forexample, in the case of the OFDM waveform used in LTE, each activesubcarrier index in the FFT preferably constitutes a dictionary 440“word,” in part or in whole. The FIG. 4 matching pursuit process maythen use a dictionary matrix made up of such FFT subcarriers asdictionary words. The construction of the dictionary for OFDM signalingproceeds as illustrated in section III of Maechler, et al., “MatchingPursuit: Evaluation and Implementation for LTE Channel Estimation,”Proc. 2010 IEEE Int. Symp. on Circuits and Systems 589-592 (2010). Otherdictionaries can be implemented for any type of signal used for amultiple access wireless scheme. Further discussion of matching pursuitand the construction of appropriate dictionaries is discussed in U.S.patent publication No. 2014/0269883, which publication is incorporatedby reference here in its entirety for all purposes including forimplementation of matching pursuit and construction of dictionaries.Since the resulting matching pursuit dictionary generally consists ofnon-orthogonal column vectors, it is well-suited for future wirelessstandards with quasi-orthogonal subchannels.

Other matching pursuit strategies are known and would also be suitablefor implementing the matching pursuit channel impulse response (MP-CIR)estimator 430. Such other strategies are selected to be consistent withthe specific conditions in the relevant wireless standard. The MP-CIRestimator 430 preferably implements the matching pursuit pseudocodediscussed above due to its simplicity among MP strategies.

Translation of pseudocode like that above into circuitry is within thecapabilities of persons of ordinary skill in this art. It will beappreciated that this procedure could be implemented through software ina processor or it could be implemented in circuitry in combination withmemory. Where desirable or advantageous, the procedures discussed herecan be implemented as hardware through, for example, a hardware designlanguage. Alternately, the above procedures can be readily implementedin a digital signal processor or other processor that processescommunication signals within a communication system. Those of ordinaryskill will appreciate that the receiver described here can beimplemented in a mixture of hardware and software elements as selectedto achieve different objectives such as computational efficiency andpower efficiency.

For a single base station and a single antenna receiver and for thedesignated LP or EP received pilot symbols 402 a stream matching pursuittime of arrival estimator 450 (FIG. 4) will provide one measurement oftime of arrival (TOA). Consequently, a single base station will producea plurality of TOA measurements consisting of at most twice as manycalculations as the number of antennas at the receiver. Given N basestations requiring TOA measurements, and labeling TOA measurements from1 to K for each base station, then the TOA for base station n (1≤n≤N)based on EP or LP calculations is,

TOA_(n,EP)=minimum(TOA_(n,1,EP),TOA_(n,2,EP), . . . ,TOA_(n,k,EP), . . .,TOA_(n,K,EP))

TOA_(n,1,P)=minimum(TOA_(n,1,LP),TOA_(n,2,LP), . . . ,TOA_(n,k,LP), . .. ,TOA_(n,K,LP))

TOA_(n)=minimum(TOA_(n,EP),TOA_(n,LP)).  EQ. 4

Equation 4 is flexible. Other formulations for TOA_(n,EP) and TOA_(n,LP)can be implemented that are appropriate for different wirelessstandards.

FIG. 5 shows another implementation of a receiver that improves therobustness and reliability of first path identification by modifying thematching pursuit channel impulse response (MP-CIR) estimator 430 of theFIG. 4 receiver to be responsive to metrics from a time domain channelestimator (TDCE) 560, resulting in the MP-CIR estimator 530. MP-CIRestimator 530 preferably also provides a more reliable measure of thefirst path time of arrival or delay. FIG. 5's queue 500, interpolator510, correlator 520, dictionary 540 and time of arrival estimator 550may be the same as those illustrated in FIG. 4. The time domain channelestimator 560 may, for example, have the form and operation described inU.S. Pat. No. 8,824,527, which patent is incorporated by reference herein its entirety.

The FIG. 5 receiver preferably includes a TDCE 560 that outputs or othercircuitry that effectively calculates a CIR estimate, CIR_(est)[m], 562that is input to the signal to noise (SNR) metric processor 570. The SNRmetric processor preferably determines a stopping_criterion_SNR[m] valuein step 3.4 of the matching pursuit pseudocode procedure implemented byMP-CIR estimator 530. The OFDM receiver symbol error rate (SER) issensitive to the accuracy of the CIR estimate and preferred receiversutilize this sensitivity to improve the performance of the matchingpursuit stopping criteria. More specifically, the MP-CIR estimator 530preferably uses a stopping_criterion_SNR[m] to stop the matching pursuititerations for time of arrival (TOA) measurements, which results in amore robust TOA measurement. Furthermore, the CIR_(est)[m] 562preferably identifies all paths in the channel present for the time ofarrival measurement. When that is done, the equation 3 SNR metric can bemodified to define the stopping_criterion_SNR[m] as the ratio of allactive paths' power to the remainder power due to noise. That is,

$\begin{matrix}{{{SNR}\lbrack m\rbrack} = {\frac{\sum_{l = {{all}\mspace{14mu}{signal}\mspace{14mu}{paths}}}{{{CIR}_{est}\lbrack l\rbrack}}^{2}}{{\sum_{l = {{all}\mspace{14mu}{paths}}}{{{CIR}_{est}\lbrack l\rbrack}}^{2}} - {\sum_{l = {{all}\mspace{14mu}{signal}\mspace{14mu}{paths}}}{{{CIR}_{est}\lbrack l\rbrack}}^{2}}}.}} & {{EQ}.\mspace{14mu} 5}\end{matrix}$

Equation 5 improves on the SNR estimate by considering all paths thathave signal, and not just the strongest path. This is particularlyadvantageous for Rayleigh fading channels, which are prominent in urbanwireless communication deployments.

In the FIG. 5 receiver, the metrics processor 570 provides metricscharacteristic of the channel impulse response (CIR) estimation,preferably calculated for each of the received symbols that hasestimation pilot symbols, as illustrated in FIG. 2. Preferably, themetrics processor at least provides a measure for the CIR's time span.The TDCE 560 preferably provides a CIR_(est)[m] 562 having a reducedtime span, which is typically about half of the cyclic prefix length,for example 72 samples in the LTE standard. This pre-selection of 72samples by the TDCE 560 offers greater reliability in the SNR metricprocessor 570. Further metrics preferably may include the receivedsignal to noise ratio (SNR) and the channel's power delay profile (PDP).Since the TDCE is calculated repeatedly during normal receiveroperation, preferably for each received symbol, these metrics can beprovided regularly to the SNR metric processor 570 to improve on itsaccuracy. FIG. 5 shows the outputs (512, 514, 522) indexed as instances[n], for n an integer. CIR_(est)[m] 562 and MP_(metric)[m] 572 areindexed as [m], for m an integer, and this denotes that they may becalculated at a different rate than variables indexed with [n].

Preferably the channel impulse response information from the TDCE 560 isused in SNR metric processor 570 for different implementations of astopping criterion as required by step 3.4 in the matching pursuitpseudocode listed above. The TDCE 560 preferably determines CIRs foreach received symbol; the MP-CIR estimator 530 is likely to measure timeof arrival (TOA) less frequently than the TDCE 560 determines CIRs orthe metrics processor 570 updates its metrics. Depending on thefrequency of CIR measurements by the TDCE and requests for TOAmeasurements, it may be desirable to best determine how to average theincreased rate of measurements by the TDCE 560 and the metrics processor570. Preferably, the output of the TDCE 560 and the metrics processor570 are accumulated and averaged in an appropriate way to providedesired inputs to the SNR metric processor 535.

The MP-CIR estimator 530 preferably identifies a first path andestablishes a first path delay or time of arrival using matching pursuitas discussed above. The time of arrival estimator 550 may, for example,establish a time reference 582 at the beginning of the slot (subframe orframe) 580 relevant to the observed first path and then identify (countfrom the time reference) the sample associated with the first path andestablish the time of arrival 552 from the sample count and the samplingrate.

FIG. 6 shows another receiver implementation that provides a matchingpursuit strategy responsive to a channel impulse response's power delayprofile (PDP). The receivers of FIGS. 4 and 5 determine a matchingpursuit channel impulse response each time the wireless network requestsa time of arrival measurement. Determining a best time of arrival valuemay require 2×K total time of arrival calculations, as illustrated byequation 2. This many occurrences of the matching pursuit channelimpulse response estimator calculations can place a demand on a mobiledevice's resources. Accordingly, a strategy that reduces the number ofmatching pursuit iterations and calculations for time of arrivaldeterminations may be preferred in some circumstances.

The matching pursuit channel impulse response (MP-CIR) estimator 530determines a channel's impulse response using inputs consisting ofcomplex valued samples over the CIR's time span. Complex valuecomputations are important for CIRs used for equalization, since phaseis an important correction for an equalizer. In detecting the presenceof a first path, the phase of each path is less likely to affect theresult. The inventor has observed that the phase is comparativelyunimportant using simulations comparing both approaches. An additionaladvantage is that the complexity in the matching pursuit implementationis reduced by using real-valued samples.

The receiver of FIG. 6 preferably has a queue 600 that accumulates theestimation pilot (EP) symbols 692 received by the first antennadesignated for time of arrival (TOA) measurements. Interpolator 610preferably reacts to these queued symbols to compute virtual EPs thusincreasing the density of estimation pilot information among all queuedEP symbols. The FIG. 6 receiver locally generates estimation pilotreference symbols based on the standard-dictated information.Alternately, the receiver can store previously generated EP referencesymbols. U.S. Pat. No. 8,824,527 provides a discussion of generatingpilot reference symbols and that patent is incorporated by reference inits entirety and for all purposes. Correlator 620 reacts to thelocally-generated reference EP symbols 691 and the queued, received EPsymbols to output the correlation between these two signals. Thus far,this is the same procedure as described for the queue 400, interpolator410 and correlator 420 in FIG. 4. This process preferably is replicatedfor each of the N antennas (1≤n≤N) in the FIG. 6 receiver, with the Nthantenna shown to have queue 602, interpolator 612 and correlator 622.Optimization preferably determines if N physical instances of thesemodules 602, 610, 620 are to be implemented, or if a time-sharing methodis feasible and so a smaller number of these circuits might be provided.

Preferably the process and circuitry described above and shown in FIG. 6for the EP symbols is replicated for the LP symbols 696, 698, with thereceiver using a locally generated reference LP signal 693, and torepresent each of N antennas (1≤n≤N). FIG. 6 shows the first antenna tohave queue 604, interpolator 614 and correlator 624 and shows the Nthantenna to have queue 606, interpolator 616 and correlator 626.

The FIG. 6 receiver determines a power delay profile (PDP) by computingthe absolute value squared 660, 662, 664 or 666 of the signal outputfrom the respective correlator 620, 622, 624, or 626 outputs. Theabsolute value operation removes the phase information, as discussedabove. Unlike the correlator outputs in FIG. 4 or FIG. 5, the FIG. 6power delay profiles can be coherently summed (since they have no phaseinformation) to improve the robustness and reliability in TOAmeasurement. Preferably the FIG. 6 receiver determines a weightedaverage 670 of the various absolute value PDP measures received by theaveraging circuitry 670. The FIG. 6 receiver may use any of variousweighted averaging strategies. Preferably, the weighted averagingcircuitry 670 uses a measure of SNR to determine the significance ofeach power delay profile 660, 662, 664, 666 it receives.

The output of the weighted average module 670 is a single power delayprofile that is an improved measure of the active paths from theparticular base station designated for the TOA measurement. Preferably,the MP-PDP estimator 630 is responsive to this single PDP estimate 672to perform matching pursuit estimation on the power delay profile toidentify the first path. The matching pursuit pseudocode, an appropriatedictionary and the associated process flow discussed above with respectto preferred implementations of the matching pursuit channel impulseresponse (MP-CIR) estimator 430 are preferably replicated for MP-PDPestimator 630, since the properties for estimating the MP-PDP aresimilar. Preferably the FIG. 6 receiver determines a dictionary 640 thatencompasses the combined information from the estimation pilot (EP) 691and the location pilot (LP) 693 reference signals with the same approachpreviously given for the dictionaries 440 and 540. Preferably then,MP-PDP estimator 630 reacts to the power delay profile estimate 672 andthe dictionary 640 to determine a final estimate of the power delayprofile or PDP, which identifies the first path. Time of arrival (TOA)estimator 650 reacts to this final PDP estimate 632 to determine thevalue of TOAn for base station n (1≤n≤N). This procedure is thenrepeated for each of the remaining base stations designated for TOAmeasurement.

Use of the FIG. 6 receiver or the method it illustrates can reduce thecomplexity of the circuitry and calculations due to the fewer instancesof matching pursuit calculations as compared to the receivers andmethods of FIGS. 4 and 5.

FIG. 7 shows another implementation of a receiver. In the FIG. 7receiver, except as noted, the circuitry has similar structure andoperation to that described for the FIG. 6 receiver and so thatdiscussion is not repeated here. Similar numbering and identifiersindicate this similar structure and operation. In FIG. 7, the matchingpursuit power delay profile (MP-PDP) estimator 730 is illustrated asalso being responsive to metrics as measured from a time domain channelestimator (TDCE) 780. The time domain channel estimator 780 and metricsprocessor 790 may be selected to be the same as, or similar to, the TDCE560 and metrics processor 570 shown in FIG. 5 and discussed above.Preferably, the TDCE 780 and metrics processor 790 of the FIG. 7receiver at least provides a measure of the channel's power delayprofile's time span. The TDCE 780 and metrics processor 790 might alsoprovide further metrics including the received SNR. Since the FIG. 7receiver determines the time domain channel estimate persistently,preferably for each received symbol, the FIG. 7 receiver has thesemetrics available to improve on the performance of the MP-PDP estimator730. As discussed above with respect to the FIG. 5 receiver, the metricscharacteristic of the channel provided by the TDCE can be incorporatedinto the stopping criterion using in step 3.4 of the matching pursuitpseudocode, which the MP-PDP estimator 730 preferably also implements.TDCE 780 preferably calculates CIRs for each received symbol, metricsprocessor 790 preferably determines metrics for each received symbol andTOA measurements are likely to be less frequent. Depending on thefrequency of CIR and metrics measurements by the TDCE and metricsprocessor and depending on the requests for TOA measurements, thoseskilled in the art can best determine how to average the increased rateof measurements and metrics by the TDCE 780 and metrics processor 790.The FIG. 6 and FIG. 7 receivers both determine the time of arrival fromthe first path identification in a manner similar to those illustratedin FIGS. 4 and 5.

The measurement for any TOA_(n) has a granularity that is dependent onthe sampling interval for the incoming signal. In the case of the LTEstandard, the interval between samples depends on the bandwidth for anygiven base station. The period or spacing between samples in LTE isT_(s)=1/30×10⁶ seconds. Light in a vacuum travels roughly ten meters inone T_(s) period (i.e., light travels a distance equal to the speed oflight times the LTE sampling period T_(i)). Fractional T_(s)measurements are needed to allow for distance granularities (resolution)of less than ten meters.

FIG. 8 shows a receiver configuration that can provide greater time ofarrival and position granularity. The FIG. 8 configuration could be usedin any of the time of arrival estimators shown in the drawings ordiscussed above by adding an interpolator 810 following the matchingpursuit estimator and other appropriate circuitry so that theinterpolator can receive multiple time of arrival signals from differentservers over different paths or through different antennas overdifferent paths. FIG. 8 specifically shows interpolator 810 preferablypositioned to receive the first path identification (650, 750) outputfrom the matching pursuit power delay profile time of arrival estimators(630, 730) shown in FIGS. 6 and 7 respectively. The MP-PDP estimator 830shown in FIG. 8 corresponds to the MP-PDP estimators 630, 730 shown inFIGS. 6 and 7 and the first path processing circuitry 850 corresponds toand performs the same functions as the first path processors 650, 750shown in FIGS. 6 and 7.

The matching pursuit power delay profile estimator 830 will likelyidentify multiple candidate paths of arrival and further identifies afirst path of arrival. Upon the identification of a first path ofarrival, the MP-PDP estimator 830 preferably identifies another path andpreferably the last path of arrival. The MP-PDP estimator 830 preferablyidentifies a first path of arrival at time t_(1st) having magnitudey_(1st) and identifies a last path of arrival at time t_(L) having amagnitude y_(L). Interpolator 810 preferably implements an interpolationscheme to estimate the amplitude y_(interp) at time t_(interp), suchthat t_(1st)>t_(interp)>t_(L). For example, linear interpolation may beused. This new value of t_(interp) is used in the first path processor(650, 750, 850) instead of t_(1st), as alternatively suggested in FIG. 6and FIG. 7. The FIG. 8 receiver may further include a weighted averagecalculator 870 similar in construction and function to the weightedaverage calculators 670 and 770 shown in FIGS. 6 and 7. Preferably thefirst path processor 650, 750, 850 receives and uses the output of theweighted average processor to weight the signals from each antenna inidentifying the first path and processing the related information.

The FIG. 8 receiver, like the other receivers discussed here (includingFIGS. 6 and 7, although it is not illustrated there), obtains the slotor subframe timing information 860 and provides the reference timeT_(subframe) 862 to the first path processor 850 to determine the timeof arrival TOA_(est) 852.

Various implementations of receivers and receiving methods have beendescribed to demonstrate measuring the time of arrival (TOA) for aspecified signal from a designated base station in a wireless network.As described, the receivers may have varying numbers of antennas and mayuse different pilot signals. Preferably these receivers and receivingmethods react to the received symbols from a wireless system exhibitingmultiple access schemes. As prescribed in the standards, the networks orbase stations can use these TOA measures to make additional measuresthat can determine receiver location in a network serviced by multiplebase stations.

The present invention has been described in terms of certain preferredembodiments. Those of ordinary skill in the art will appreciate thatvarious modifications and alterations could be made to the specificpreferred embodiments described here without varying from the teachingsof the present invention. Consequently, the present invention is notintended to be limited to the specific preferred embodiments describedhere but instead the present invention is to be defined by the appendedclaims. Other systems, methods, features or advantages will be or maybecome apparent to those skilled in the art of wireless receivers,networks and measurements required by implementations of a standard.

What is claimed is:
 1. A method of determining time of arrival in awireless network, comprising: receiving at a receiver from a wirelessnetwork a request for time of arrival information; extracting locationpilot symbols from a received signal; identifying a first pathassociated with the location pilot symbols using matching pursuit; anddetermining a time of arrival based on the first path identified usingmatching pursuit estimation.
 2. The method of claim 1, wherein theextracting location pilot symbols comprises comb filtering in afrequency domain.
 3. The method of claim 1, wherein the identifying afirst path uses both location pilot symbols and estimation pilotsymbols.
 4. The method of claim 3 further comprising comb filtering toextract estimation pilots.
 5. The method of claim 3 further comprisinginterpolating from received location pilot symbols to generate virtuallocation pilot symbols.
 6. The method of claim 1 further comprisingdetermining a signal to noise measure for a channel impulse response andstopping matching pursuit estimation in response to the signal to noisemeasure.
 7. The method of claim 6, wherein the received signal has anOFDM waveform.
 8. The method of claim 1 further comprising correlatingbetween a plurality of received location pilot symbols and a referencesignal and providing a result of the correlating as an input to thematching pursuit estimation.
 9. The method of claim 8, wherein thematching pursuit estimation uses location pilot symbols and estimationpilot symbols.
 10. The method of claim 9 further comprising determininga signal to noise measure for a channel impulse response and stoppingmatching pursuit estimation in response to the signal to noise measure.11. The method of claim 10 further comprising interpolating fromreceived location pilot symbols to generate virtual location pilotsymbols.
 12. The method of claim 1, further comprising interpolatingbetween two or more sets of path information to generate a time ofarrive with a granularity smaller than defined by the system samplingperiod.
 13. The method of claim 1, further comprising determining apower delay profile for a signal comprising plural location pilotsymbols and performing matching pursuit estimation using the power delayprofile.
 14. The method of claim 13, wherein the matching pursuitestimation is performed using a location pilot power delay profile andan estimation pilot power delay profile.
 15. The method of claim 14,further comprising generating virtual location pilot symbols andgenerating virtual estimation pilot symbols.
 16. The method of claim 13,further comprising interpolating between two or more sets of pathinformation to generate a time of arrive with a granularity smaller thandefined by the system sampling period.
 17. A method of determining timeof arrival in a wireless network, comprising: receiving at a receiverfrom a wireless network a request for time of arrival information;extracting pilot symbols from a received signal; identifying a firstpath associated with the extracted pilot symbols using matching pursuit;and determining a time of arrival based on the first path identifiedusing matching pursuit estimation.
 18. The receiver of claim 16, whereinthe extracted pilot symbols comprise extracted location pilot symbols.19. The receiver of claim 16, wherein the extracted pilot symbolscomprise extracted estimation pilot symbols.
 20. The receiver of claim16, wherein the extracted pilot symbols comprise extracted locationpilot symbols and extracted estimation pilot symbols.
 21. The method ofclaim 20, further comprising interpolating between two or more sets ofpath information to generate a time of arrive with a granularity smallerthan defined by the system sampling period.
 22. A receiver having a timeof arrival function, the receiver comprising: memory coupled to storeextracted pilot symbols; an interpolator coupled to receive theextracted pilot symbols and to generate virtual pilot symbols; amulti-symbol correlator that correlates the extracted pilot symbols andthe virtual pilot symbols with a reference signal and outputs an initialpilot channel impulse response; a matching pursuit estimator providing achannel impulse response estimate responsive to the initial pilotchannel impulse response; and a time of arrival estimator responsive tothe matching pursuit estimator to identify a first path and to determinea time of arrival for the first path.
 23. The receiver of claim 22,wherein the extracted pilot symbols comprise extracted location pilotsymbols.
 24. The receiver of claim 22, wherein the extracted pilotsymbols comprise extracted estimation pilot symbols.
 25. The receiver ofclaim 22, wherein the extracted pilot symbols comprise extractedlocation pilot symbols and extracted estimation pilot symbols.
 26. Thereceiver of claim 15, further comprising a time of arrival interpolatorcoupled to receive path information from matching pursuit estimator, thetime of arrival interpolator adapted to provide interpolated pathinformation to the time of arrival estimator so that the time of arrivalestimator generates a time of arrival with a granularity smaller thandefined by the system sampling period.